Each pin is a saint. Numbered circles are clusters: click or tap one to zoom in. Select a pin to open that saint's details. Group entries (such as the 21 Coptic Martyrs) list their members; select a name to see that person, then use the back button to return to the group.
Open Filters (funnel icon) and use the Base Map slider. It switches the map to how the world looked in different eras, up to Present Day, and fills in Filter by Year with that era so only the saints who lived then are shown. Present Day shows every saint, and you can still adjust the years by hand.
Open Search (magnifying glass), type a name or religious order, then press Search or Enter. Select a result to jump to that saint. Search only covers saints that match your current filters. Reset clears the search.
In Filters, tick Layers and Status options to show only those saints. Filter by Year narrows saints by date: drag the two handles (BC years move in steps of 10, AD years in steps of 1) or type a start and end year. Reset Selection clears everything.
Zoom in close on certain buildings, such as St. Peter's Basilica, and a floor plan appears over the map. If a building has several floors, use the floor button on the map to switch between them.